Toxoplasma gondii: characterization of a monoclonal antibody recognizing antigens of 36 and 38 kDa with acid phosphatase activity located in dense granules and rhoptries.

Acid phosphatase activity in Toxoplasma gondii was studied by electron microscopy and found in the rhoptries and dense granules. On Western blots, six bands were found to have acid phosphatase activity; a double band of 36-38 kDa showed the strongest activity. In order to raise monoclonal antibodies to antigens of potential importance in parasite invasion of the host cell, mice were immunized with live T. gondii of the cyst-forming ME119 isolate and later boosted with tachyzoites of the RH strain. One of the monoclonal antibodies, S16, recognized the 36- to 38-kDa antigen. Immunogold labeling and electron microscopy showed that the antigens were located in dense granules and rhoptries. Using immunoprecipitation with S16, SDS-PAGE, IEF, and Western blotting, we show that the 36- to 38-kDa rhoptry antigen recognized by S16 has acid phosphatase activity.